EFFECT OF MUSCLE FATIGUE ON THE ELECTROMYOGRAM CHARACTERISTICS OF THE LUMBAR PARASPINAL AND HIP EXTENSOR MUSCLES

Abstract

PURPOSE: The purpose of this study was to determine the effect of fatiguing trunk extension exercise on the electromyographic (EMG) characteristics of the lumbar paraspinal and hip extensor muscles. METHODS: Twenty healthy volunteers (22.5 years +/− 2.8) participated in the study. Subjects upper body mass (UBM) was measured using a digital bedside scale. Next, subjects performed a maximal voluntary isometric contraction on a Roman chair positioned at 15 degrees relative to horizontal. Subjects were then loaded at an intensity representing 50%, which consisted of their UBM and an added load. To induce muscle fatigue, subjects performed dynamic trunk extensions to muscular failure on the Roman chair. Surface EMG signals were recorded from the right lumbar paraspinal (L4-L5), gluteus maximus and biceps femoris muscles. Root mean square EMG was determined for the concentric portion of each contraction, and all values were normalized to the first contraction and expresed as a percent of fatigue. RESULTS: Polynomial regression analysis revealed a quadratic curve in normalized EMG for the lumbar extensors over percent of fatigue (R2 = .0807, SEE = .228; β2 = −8.245-5; breakpoint = 57.9%) (p < 0.000), and for the gluteus maximus over percent of fatigue (R2 = .5059, SEE = .865; β2 = .00017; breakpoint = 35.9%) (p < 0.014). The biceps femoris demonstrated a significant linear increase in normalized EMG over percent of fatigue (R2 = .4667, SEE = .284; β = .0091) (p < 0.000). To further investigate the decreased EMG response of the lumbar extensors associated with fatigue, subjects were analyzed individually for a quadratic fit and breakpoint. Results revealed that 68.5% of subjects demonstrated a significant quadratic bend in their lumbar EMG over percent of fatigue (breakpoint = 53.01% + 17.6). CONCLUSION: During fatiguing trunk extension exercise an increase in lumbar paraspinal EMG occurs up to approximately 55% of maximum, at which point a decrease in EMG is observed. It appears that as the lumbar muscles become fatigued, the gluteus maximus is activated to allow for continuation of the exercise.